Polyesters such as poly(glycolide), poly(lactide), and poly(caprolactone) are biocompatible and biodegradable polymers that are often used in biomedical applications such as drug-delivery. Homopolymers of glycolide, lactide, or caprolactone can oftentimes be too hydrophobic in aqueous environments and can also be difficult to covalently functionalize, which limits their use for particular applications. Efforts have been made to prepare synthetic analogs of glycolide, lactide, and caprolactone in an attempt to overcome these limitations. Examples include cyclic esters having unsaturated functional groups that can react to provide modified cyclic esters or modified poly(cyclic esters) that have a desired property suitable for the application at-hand.
A method for preparing cyclic esters having unsaturated functional groups is disclosed in U.S. Patent Application Publication No. 2009/0054619 to Baker et al. According to the method, an alkynyl substituted α-hydroxy acid (alkynyl-substituted glycolide) is condensed in a dilute solution to form the cyclic ester. The resulting cyclic ester or polymer prepared from the cyclic ester can be functionalized, for example through “click chemistry,” to provide a poly(cyclic ester) having a functionality tailored for a particular application. This process is limited, however, inasmuch as the reaction of the α-hydroxy acid to form the cyclic ester requires a lot of solvent and is low yielding and therefore not optimal for use in a cost-effective or industrial scale process.
Accordingly, a need exists for improved methods for preparing cyclic esters having unsaturated functional groups that overcome the aforementioned limitations. A need also exists for improved polyesters that allow for efficient functionalization that can allow for hydrophilic functional groups to be introduced onto the polymer to thereby increase the hydrophilicity of the polymer.